Strömungsabhängigkeit der Lochkorrosion eines Cr-Ni-Stahles in NaCl-Lösung. Teil I: Versuche mit der rotierenden Scheibe

Flow dependence of the pitting of a CrNi steel in NaCl solution. Pt. I: Experiments with the rotating disc The pitting of the CrNi steel (German designation X 5 CrNi 18 9, Werkstoff Nr. 1.4301) in NaCl solution (1 and 3 N) at 25 °C has been investigated with the aid of the rotating disc, with a view to its dependence from the number of revolutions and the thickness of the diffusion layer. Potentiokinetic and potentiostatic experiments have shown, that the pitting potential for stable pitting is a function of flow in a more pronounced way than the potential for repassivating pitting corrosion. The pitting density increases as the number of revolutions increases and the thickness of the diffusion layer decreases, while the corrosion limiting current density and the average and maximum pit radii decreases in the same conditions. This goes to show that the sit locations are repassivated when exceeding a critical size which decreases with increasing number of revolutions. Contrary to the conditions prevailing in stagnant solutions where the pit density is almost independent from time, new pits are generated permanently despite repassivation under conditions of flow. Consequently, there is a apparent increase of pit density with time.     

Mikrobielle Werkstoffzerstörung – Simulation,Schadensfälle und Gegenmaßnahmen für metallische Werkstoffe: Manganoxidierende Bakterien und Lochkorrosion an Turbinenteilen aus CrNi-Stahl in einem Laufkraftwerk

Microbial deterioration of materials – simulation, case histories and countermeasures for metallic materials: Manganese oxidizing bacteria and pitting of turbine components made of CrNi steel in a hydroelectric power plant In a hydroelectric power plant pitting corrosion of the turbine runner blades (material G-X 5 CrNi 13 4) and the discharge ring (X 3 CrNi 13 4) ocurred at chloride levels of 170 mg/l maximum. Both, potentiostatic stepwise experiments and immersion tests gave no indication for susceptibility to pitting or crevice corrosion of this material in such water. Analysis of calcareous deposits found on the metal surface revealed high amounts of manganese dioxide. It is shown experimentally, that the CrNi-steel can undergo chloride induced pitting corrosion if polarized by manganese dioxide. Manganese oxidizing bacteria and possibly fungi were found to form the deposits. This case may be considered as microbiologically influenced corrosion (MIC) under aerobic conditions.     

Beeinträchtigung der Lochkorrosionsbeständigkeit von CrNi-Stählen durch dünne Oxidschichten

Influence of thin oxide films on pitting corrosion of CrNi steels Chloride-induced pitting corrosion of stainless steel (Materials No. 1.4301 and 1.4571) was investigated with the aid of chronopotentiostatic tests. Oxide films upon the surface (temper colours), addition of inhibitors (sulphate and nitrate) and temperature were the testing variables. Two different critical pitting potentials have been found, which give information on the potential ranges for stable passivity, latent and stable pitting corrosion. The results only indicate a small effect of Mo content of the material and test temperature. The inhibitors (sulphate and nitrate) have a marked effect. Specimens with a clean surface produced by pickling are markedly more resistant against pitting corrosion than specimens with a yellow temper colour, whereas the difference in corrosion resistance between yellow and blue films is relatively small. The width of the critical potential range for pitting corrosion generally increases with increasing oxide film thickness.     

Der Einfluß von Schwefeldioxid,Schwefelwasserstoff und Kohlenmonoxid auf die Lochkorrosion von austenitischen Chrom-Nickel-Stählen mit bis zu 4 Massen-% Molybdän in 1 M Natriumchlorid-Lösung

The Influence of SO₂, H₂S and CO on Pitting Corrosion of Austenitic Chromium-Nickel Stainless Steels with up to 4 wt. % Molybdenum in 1 M NaCl Active corrosion of chromium-nickel stainless steel X 5 CrNi 189 (AISI 304) in H₂SO₄ is stimulated by H₂S as well as by SO₂ (extension of the potential range of active corrosion, increase of the maximum corrosion rate in the active state and of the passivation current density), but is inhibited by CO (decrease of both maximum active corrosion rate and passivation current density). It is investigated whether likewise stimulating and inhibiting effects are valid also in case of pitting corrosion of austenitic stainless steels with molybdenum contents ranging from about zero (material no. 1.4301) to 4 wt. % (material no. 1.4449), tested in 1 M NaCl (ambient temperature) saturated with the gases mentioned above. The pitting corrosion behaviour of the materials investigated is judged by their pitting potentials measured by potentiostatically controlled experiments (testing time 24 hrs). The pitting potentials are compared with those measured in 1 M NaCl, N₂-bubbled. Pitting corrosion is stimulated by SO₂, CO and H₂S, with the stimulating efficacy increasing in the sequence given before. No stimulation is found only in 1 M NaCl, SO₂-bubbled,-with the highest Mocontent. In all other cases, stimulation of pitting corrosion increases with increasing Mo-content of the stainless steels. In coarse approximation, the critical limiting potentials of stable pitting in 1 M NaCl, bubbled with H₂S, SO₂ CO, correspond to the critical potentials of repassivating pitting corrosion found in N₂-bubbled 1 M NaCl. The chemical reactions and reaction products of SO₂ in aqueous solution are discussed. The nature of the stimulating component of the corrosive medium is not quite clear. The stimulating effects of SO₂ or one of its reaction products and of H₂S on pitting are in keeping with their stimulating effect on active corrosion of the steels investigated. The stimulating effect of CO, however, is in contradiction to the results expected and cannot be explained.     

Über den Zusammenhang zwischen Lochkorrosion und Festigkeitsverhalten austenitischer Chrom-Nickel-Stähle

Relations between pitting corrosion and mechanical strength of austenitic chromium nickel steels The shape, depth and size as well as the frequency of pits formed under potentiostatic conditions on the surfaces of chromium-nickel steels (German designations X 5 CrNi 18 9 and X 5 CrNiMo 18 10) in 3% NaCl are studied with reference to stress mechanical effects of pitting corrosion. Comparative studies of pitting corrosion and strength behaviour of specimens at- tacked under pitting enable the residual tensile strength to be represented as a function of potential, and the tensile strength as such to be plotted as a function of a corrosion index containing pit depth and number. External effects like thermal treatment, alloy composition and cold deformation can be interpreted uniformly on the basis of all these graphical representations.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

Elektrochemische Untersuchung der Lochkorrosion chemisch beständiger Stähle

Electrochemical investigation into the pitting corrosion of corrosion resistant steels In view of the fact that the characteristics termed pitting potential depend from the measuring methods and nonstationary defects and hysteresis phenomena produced thereby these condition have been investigated on three different systems. In sulphate inhibited chloride solution an actual hysteresis is found with two critical boundary potentials for pit formation and pit passivation. The first of these potentials may be found either by potentiocinetic or by potentiostatic methods, the second only galvanostatically. In the case of 35% Cr steel no hysteresis is found; in this case there is no connection between pitting potential and the current and potential variations found during the investigation. With this material pitting corrosion may occur below the pitting potential while the pit passivation potential is a function of the properties of the measuring circuit. The pitting behaviour of austenitic CrNi steels containing various amounts of molybdenum may be characterized potentiostatically or potentiocinetically, but at low voltage feed rates only.     

Die Lochkorrosionsanfälligkeit des Vergütungsstahles X20Cr13 in Natriumchloridlösungen

Pitting corrosion susceptibility of the heat treatable steel X20Cr13 in sodium chloride solutions For blades in steam trubines it is common to use the heat-treatable steels with 13% Cr (X20Cr13). Cracking of blades in the region of the turbine, where the steam starts to get wet, have often been referred to be caused by corrosion fatigue. The fatigue cracks sometimes start at inclusions in the material, but more often at pits caused by pitting corrosion. Pitting is brought about by chloride containing blade deposits. To examine the pitting behaviour of the steel in NaCl-solutions we measured the mixed-potential, the potentiostatic piting potential, the mass-loss and the pits per area as depending on temperature (20–80°C), pH (5–7–9), oxygen content (20 μg/kg up to saturation with oxygen) and chloride content (8×10^?5 mole/l up to saturation with NaCl 5,4 mole/l). The results show that steel X20Cr13 suffers pitting corrosion because of it's low Cr content in all technical possible NaCl solutions down to a Cl content of 10^?5 mole/l at the mixed potential.     

敏化对不锈钢孔蚀性能的影响

用电化学测试技术研究了不同敏化处理的304不锈钢的耐孔蚀性能,探讨了Cl-浓度、温度和pH值对孔蚀电位Eb的影响。结果表明,敏化热处理导致Eb值下降;随Cl-浓度增加,介质温度升高,pH下降,使Eb值变负;不锈钢的晶间腐蚀使耐孔蚀性能变劣     

Loch- und Spaltkorrosion von Chrom- und Chromnickelstählen in chloridhaltigen Lösungen

Pitting and crevice corrosion of stainless steels in chloride solutions In practice stainless steels in chloride containing waters are found to be susceptible to crevice corrosion and pitting. Corrosion tests were carried out on AISI 304 L stainless using a simulated crevice and the compositions of the electrolyte in the crevice determined. Long term potentiostatic tests were used to determine the critical potentials for crevice corrosion (U_S), for various steels in sodium chloride solutions at different concentrations and temperatures. The steels studied were 22 CrMo V 121, X 22 CrNi 17 and AISI 304 L. Like the critical pitting potential (U_L), U_S was found to have a strong dependence on the chloride content of the external solution. At higher concentrations the two potentials were similar. At lower concentrations the U_S was lower than U_L. The knowledge of these critical potentials together with well known rest potentials for a steel in an electrolyte of known concentration, allows conclusions to be drawn about its susceptibility to pitting and crevice corrosion. The method is suitable also for other passive metals.     

Zum Einfluß von Mangan auf die Korrosionseigenschaften von austenitischen 18.10-CrNi-Stählen

Influence of manganese on the corrosion properties of austenitic 18.10-CrNi stainless steels The influence of manganese in the range of 0.25 to 1.5 mass-% on the passivation and pitting corrosion behaviour of unstabilized and Tistabilized austenitic 18/10 CrNi stainless steels is examined by determination of useful characteristical electrochemical dates using potentiodynamical polarization measurements in H₂SO₄-acidic and neutral model electrolytes. In the case of Ti-stabilized steels, a trend to an improved ability to passivation and to an increased pitting corrosion resistance is signified with increased Mn-content. This is in agreement with the austenite stabilizing effect of manganese. In the case of unstabilized steels, a significant deterioration of the passivation and pitting corrosion behaviour is observed, if the Mn-content of the steel is increased from less than 0.7 to more than 1.0 mass-%. These observations are discussed in the viewpoint of segregation of Mn-rich sulfide inclusions in the steel, which are essentially influenced by the presence of titanium in the steel.     

Der Einfluß von Stickstoff auf die korrosionschemischen Eigenschaften lösungsgeglühter und angelassener austenitischer 18/10 Chrom-Nickel-und 18/12 Chrom-Nickel-Molybdän-Stähle III. Loch-und Spannungsrißkorrosionsverhalten in wässerigen Lösungen von NaCl und MgCl2

Influence of nitrogen on the corrosion prop-erties of solution treated and annealed aus-tenitic 18/10 CrNi and 18/12 CrNiMo steels.III. Pitting and stress corrosion cracking in aqueous solutions of NaCl and MgCl₂ Nitrogen in quantities from 0.022 to 0.138 and 0.028 to 0.130% has just a negligible (though favourable) influence on the pitting corrosion behaviour of the CrNi steels 18 11 and the CrNiMo steels 1713 2.6 in different heat treatment states. the pitting potential is then displaced to more positive values. the mo bearing steels on the other hand, after extended heat treatment at high temperatures are characterized by intercrystalline corro-sion susceptibility of the piting type, even when the material without heat treatment is to be considered as resistant in the Strauß-test. this fact may be attributed to the pre-cipitation of Laves and Fe₂Mo phases giving rise to Mo depletion. No influence whatever, however, was found in the case of ?-, σ- and Z-phase precipitation since these to not produce chromium depletion in the matrix. The transcrystalline corrosion resistance of these steels is not influenced by nitrogen. Des teneurs en nitrogène(0,022-0,138 et 0,028-0,130% resp.) n'out qu'une influence assez faible (bien que favourable) sur le com-portement à la piquration des aciers CrNi 18/11 et CrNi/mo17 13 2,6 resp. après traite-ments diffèrents thermiques. Le potential de piqûration est alors déplacé vers des valeurs plus positives. Les aciers au Mo, d'autre part, sont caractérisés, après traitement thermique prolongé à des temperature élevées, par une sensiblité à la corrosion par piqûration in-tercristalline, même si le matériau avant traitement thermique n'est opas sensible dans l'essai de strauß. On peut attribuer ce fait à la précipitation des phases Laves et fe₂Mo qui donne lieu à un appauvrissement en Mo. aucune influence n'aété constatée dans le cas des précipitations des phases ? σ et Z qui n'entrainent pas un appauvrissement en Cr dans la matrice. La résistance à la cor-rosion transcritalline de ces acters n'est pas influencée par le nitrogène.     

Einfluß des Kriechens auf die Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißer 35%iger Magnesiumchloridlösung

Creep effect on stress corrosion cracking of austenitic CrNi steels in boiling 35% magnesium chloride solution Potentiostatic and potentiodynamic polarization curves of steel X 5 CrNi 18 9 in 35% MgCl₂-solution at 120° C do not show significant differences. Important for SCC tests is a narrow potential region before the onset of the potential of pit nucleation. SCC-experiments were carried out using two different techniques:

• (a) loading in the electrolyte
• (b) prestraining in air at 120° C; after transient creep the SCC test was initated.

Applying the technique described under (b) a decrease of SCC-susceptibility according to the crack nucleation expected in the slip dissolution model does not take place. Accordingly the dependence of time to failure on potential as well as on stress is found to be almost similar. Only in the absence of pitting a SCC threshold stress exists with its value in the range of yield strength. On the other hand SCC failure in the elastic region (< 0.1 σ_y) is observed when crack nucleation starts in corrosions pits.     

Effect of Surface Finish on the Pitting Corrosion Behavior of Sensitized AISI 304 Austenitic Stainless Steel Alloys in 3.5% NaCl Solutions

The effect of surface condition down to 120 and 1000-grit finish, corresponding to 1.47 and 0.06 microns RMS (Root Mean Square), respectively, on the pitting corrosion behavior of sensitized and mill-annealed AISI (American iron and steel institute) 304 stainless steel was studied in 3.5% NaCl solutions at 23 and 50°C by electrochemical methods. The polarization curves have revealed a clear dependence of pitting corrosion on the surface finish, on the degree of sensitization, as well as on the test temperature. Surface condition has made a significant contribution to pit initiation in that the pitting potential was lowered as the surface roughness increased. The deleterious effect of surface roughness on the pitting potential of the AISI 304 stainless steel alloy in 3.5% NaCl is more pronounced on sensitized samples and becomes more evident with increasing sensitization time and test temperature.     

Schnelle Lochkorrosion durch ungewöhnliche Anodenstabilisierung in einem Stahl/Wasser-System

Fast pitting corrosion in a steel/water system as a result of unusual stabilization of anodes Corrosion damage as a result of fast pitting was observed in industrial equipment made of unalloyed steel. The rate of formation and the manifestation of the pits are similar to pitting corrosion of stainless steels. Estimation of the kinetics of the localized corrosion in terms of differential aeration cells led to a model which indicates the stabilization of small anodic regions. The stabilization was caused by precipitation of the products of reaction between ferrous ions and polymeric silicate. The high rate of pitting also suggests that the oxygen entering the system during service periods involving the exposure of the system to air has enhanced the corrosion. These ideas could be substantiated by experiments, which also revealed particularly the effect of the concentration of NaCl. Anodic passivation by the addition of Na₂CO₃ or perhaps inhibition may be considered as candidate protective measures.     

Oberflächenpotentialprofile von metallen

Surface potential profiles of metals Using a microcapillary method in connection with simultaneous metallographic observation surface potential profiles have been determined with the potential probe in alcoholic acid solutions on cast iron, CrNi aciers steels 188, Cr steel with 17% Cr (welded) and an AlCu alloy. It was possible to distinguish cementite from ledeburite, not however, from austenite. The potential of 188 steel under pitting conditions decreases as a linear function of pit depth. The appearance of potential minima in a weld of ferrite steels is evidence of a intercrystalline corrosion in that particular medium. The precipitation of CuAl₂ in AlCu alloys at 150 °C at the grain boundaries forms an element with the grain boundary as the anode giving rise to intercrystalline corrosion.     

Assessment of stress corrosion crack initiation and propagation in AISI type 316 stainless steel by electrochemical noise technique

Electrochemical noise (EN) analyses, using time domain, frequency domain and statistical parameters, were carried out on the potential and current data obtained on AISI type 316 stainless steel during (i) pitting corrosion in deaerated 0.5 M sodium chloride (NaCl) solution, and during (ii) stress corrosion cracking (SCC) in boiling acidified NaCl solution. Visual records and statistical analysis of the current and potential data alongwith the spectral estimation using maximum entropy method (MEM) gave useful informations on differentiating between these corrosion processes and on the effect of stress in accelerating pitting corrosion. The results correlated well with the optical microscopic observations.     

304 不锈钢在氯化钠介质中点蚀缓蚀剂的研究

目的研究钼酸钠、葡萄糖酸钠及其复配物在氯化钠介质中,对304不锈钢点蚀的缓蚀作用。方法对钼酸钠、葡萄糖酸钠按不同配比进行复配得到不同缓蚀剂,采用极化曲线法分别测试在这几种缓蚀剂存在的条件下,304不锈钢在3.5%(质量分数,后同)NaCl溶液中的点蚀电位。结果单组分的钼酸钠、葡萄糖酸钠对在3.5%NaCl介质中的304不锈钢点蚀有一定的抑制作用,且两种缓蚀剂有明显的协同缓蚀效应。结论当复配缓蚀剂配比为c(钼酸钠)∶c(葡萄糖酸钠)=2∶1时,其缓蚀效果达到最佳,点蚀电位为436 mV。     

碳钢在两种氯化物溶液中的极化行为

利用极化试验和交流阻抗试验研究了碳钢在0.5MNH4Cl和0.5MNaCl溶液中的点蚀行为;通过金相显微镜和电子探针对钢的腐蚀形貌进行观察。结果表明:碳钢在0.5MNaCl和0.5MNH4Cl溶液中的极化曲线都出现活化特征。随着溶液pH值的增加和NaHCO3的添加,碳钢在NH4Cl和NaCl溶液中极化曲线上均有活化、活化-钝化转变、钝化和点蚀特征。显微观察表明,钢中的夹杂物是主要的点蚀诱发源。碳钢在0.5MNaCl和0.5MNH4Cl溶液中形成的活化峰分别与形成沉积膜和络合物有关。     

The influence of Mn compared to that of Cr,Mo and S on the resistance to initiation of pitting and crevice corrosion in austenitic stainless steels

A large number of production and laboratory heats in grades AISI 304 and 316 with normal and extremely low managanese and sulphur contents and a number of production heats in more highly alloyed austenitic stainless steels have been studied with regard to their resistance to initiation of pitting and crevice corrosion at various temperatures. The criteria for resistance to initiation was the potentiodynamic pitting potential in 0.1 M NaCl and synthetic seawater and the time to attack initiation for crevice corrosion in 0.5 and 5% NaCl solutions. A large number of production and laboratory heats in grades AISI 304 and 316 with normal and extremely low managanese and sulphur contents and a number of production heats in more highly alloyed austenitic stainless steels have been studied with regard to their resistance to initiation of pitting and crevice corrosion at various temperatures. The critieria for resistance to initiation was the potentiodynamic pitting potential in 0.1 M NaCl and synthetic seawater and the time to attack initiation for crevice corrosion in 0.5 and 5% NaCl solutions. The main aims of the study were to examine both the effect of manganese relative to that of chromium, molybdenum and sulphur and the effect of heat treatment and sulphide composition on steels with low manganese contents. Mathematical models for calculation of the pitting potentials have been constructed and multiple linear regression analysis gave the equations and their reliabilities. Lowering of the Mn content in austenitic stainless steels to 0.2% gives rise to a material of interest for constructions where pitting or crevice corrosion are judged to be the only potential types of attack, where operational disturbances leading to greatly increased corrosivity do not occur, where attack can not be tolerated, and where steel with normal managanese content has not exhibited fully satisfactory corrosion resistance. If the above conditions are fulfilled the low manganese content can be said to correspond to the same positive effect as is obtained by an addition of the least 1.5% Mo.